Great extinction came in phases

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A joint UK-Chinese team tell Nature magazine the disaster that befell the planet 250 million years ago must have happened in phases. Their conclusion is based on the abundance of "organic fossils" found in rocks at Meishan in southern China. These suggest there were at least two episodes to the mass die-off that saw up to 95% of lifeforms disappear... The new data from China supports this view. It is based on the traces left in rocks by cyanobacteria. These photosynthetic, mostly single-celled organisms existed in vast blooms in the Permian oceans. They are one of the major groups of phytoplankton, which form the basis of the marine food chain. However, the phytoplankton not eaten by higher organisms would have fallen to the seafloor over time to be incorporated into the sedimentary rocks we see today. And chemical components in their cell membranes have left telltale signs of their past existence. Specifically, a lipid molecule, known as 2-methylhopane, has left ring structures in the Meishan rock. "These ring structures are the 'hydrocarbon skeleton' - that is how we would refer to them - and they can be preserved for a very long time," explained Dr Pancost. The research team sees two peaks of abundance in the Chinese rocks which are believed to indicate periods immediately following biotic crises in the oceans - times when the collapse of higher marine lifeforms allowed the cyanobacteria populations to boom. "What we think happened was that the grazing pressure changed," explained Dr Pancost. "A lot of the fauna that went extinct went through larval stages that would have fed on the phytoplankton. "Changes in the faunal assemblages would have changed predation patterns, and this led to the phytoplankton prospering."